1. Field of the Invention
This invention generally relates to IEEE 802.11 communications and, more particularly, to a system of method for controlling beacon timing when a communications network includes both 802.11 and 802.11e participants.
2. Description of the Related Art
To minimize contention on a shared communication link or channel, IEEE 802.11 participants are synchronized to operate with respect to a common clock. Clock synchronization is maintained with the use of a beacon signal sent at target beacon transmission time (TBTT). That is, the beacon is a special management frame sent from an access point to synchronize station local timers, and to deliver protocol-related parameters. Normally, the beacon is a signal that represents the start of the contention free period. An access point/hybrid coordinator/point coordinator (AP/HC/PC) schedules a beacon for transmission when it has been determined that the medium (channel) has been idle for at least a point coordination function interframe space (PIFS). However, problems associated with the TBTT may occur if legacy 802.11 stations are participating in the network.
FIG. 1 is a timing diagram illustrating a beacon delay that occurs as a result of a 802.11 station transmitting during the contention period (CP). From the legacy IEEE 802.11 specification, non-AP stations may start their transmissions, even if the MAC surface data unit (MSDU) delivery cannot be finished before the upcoming TBTT. The rules associated with CP transmissions do not permit a participant to transmit, if the participant senses that the channel is in use. Thus, if a station is transmitting during the scheduled TBTT, the AP cannot transmit a beacon. With respect to the point coordinator function (PCF), the transmission of time-bounded MSDUs, delivered in the contention free period (CFP), is delayed if the beacon frame is delayed. Further, with respect to the hybrid coordinator function (HCF), if the beacon gets delayed in HCF, the schedule and the pending transmission in AP/HC are also delayed. The AP may be forced to change the schedule for HCF transmission in the whole beacon interval. These delays may severely affect the quality of service (QoS), as delays introduce temporal unpredictability, depending on the frame lengths, fragmentation, and the traffic. For example, the unpredictable delays introduced by the above-mentioned problem are considered unacceptable for high quality audio-visual (AV) transmissions as well as the voice applications. These beacon delays can be larger than 20 milliseconds, which results in a noticeable jitter.
In the 802.11e specification, this issue is solved in a QoS basis service set (QBSS) through the use of 802.11e-based QoS stations (QSTAs). If a QSTA cannot finish the complete transmission sequence (including ACK, if ACK is desired) before the end of a transmission opportunity (TXOP), it is not required to send the transmission. Also, the QSTAs may not send a transmission, if it cannot finish the transmission sequence before the time of the next scheduled TBTT.
However, the above-mentioned beacon non-interference rule only applies the QSTAs. Legacy (802.11) STAs do not follow this rule. Thus, in a mixed QBSS, which includes even one legacy STA, there is a chance that the beacon can be delayed. The QoS AP (QAP) may choose to disassociate the STA from the QBSS in some, but not all, circumstances. For example, a legacy STA should be able to gain some level of connectivity in a Hotspot network.
It would be advantageous if the QoS of a QBSS could be maintained when the network includes both 802.11e QSTAs and 802.11 (legacy) STAs.
It would be advantageous if a means could be found of preventing a 802.11 STA from delaying the beacon in a QBSS.